If you try renaming a file in your project, you might notice this weird behaviour when checking its status:
$ git status
On branch master
Your branch is up-to-date with 'origin/master'.

Changes not staged for commit:
(use "git add/rm ..." to update what will be committed)
(use "git checkout -- ..." to discard changes in working directory)

deleted: README.md

Untracked files:
(use "git add ..." to include in what will be committed)

README-NEW.md

no changes added to commit (use "git add" and/or "git commit -a")

Why is this happening?

The short answer is: Git relies on names to know if whether a file is tracked or not.
When you rename a file, Git believes the original file got deleted and suddenly there’s a new file in which it knows nothing about. Hence the untracked status.

Alternative: rename with Git

If you want to avoid this behaviour, you could simply use the native Git command: git mv
$ git mv README.md README-NEW.md
$ git status
On branch master
Your branch is up-to-date with 'origin/master'.